Historically, there have been three general ways to produce polyimide foams. According to the process described in Lavin et al. U.S. Pat. No. 3,554,939, a monomer mixture composed of an ester of benzophenone tetracarboxylic acid and a polyamine, the mixture having a volatile content (defined as percent weight loss in 10 minutes at 300.degree. C.) of at least 9%, is heated to a critical temperature at which foaming occurs contemporaneously with polymerization of the tetracarboxylic and polyamine components until the polyimide foam is formed.
In another procedure, described by Gagliani in Final Report NAS 9-14718 entitled "Fire Resistant Resilient Foams" dated February 1976, a mixture of diamines is added to an alcoholic solution of the half ester of benzophenone tetracarboxylic acid and reacted at 158.degree.-167.degree. F. (70.degree.-75.degree. C.) to form a heavy syrup. This syrup is heated in a circulating air oven at 180.degree. F. (82.2.degree. C.) for about 12-16 hours and the in a vacuum oven at 176.degree.-194.degree. F. (80.degree.-90.degree. C.) for 60-90 minutes, producing a polyimide precursor. Thereafter, the polyimide precursor is pulverized into a powder which is spread over aluminum foil on an aluminum plate and heated at 600.F (315.6.C) in an oven for 30 minutes to produce the foam. In a similar procedure reported by Gagliani et al. in Final Report NAS 9-15050 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Aircraft and Spacecraft" dated June 1977, the dried precursor powder formed in about the same manner was subjected, inter alia, to multi-stage heating, in which the powder was placed in a pressure vessel positioned within an oven preheated at 450.degree. F. (232.degree. C.) and held at this temperature at a reduced pressure (19.9-9.9 inches of Hg) for I5-30 minutes. The resulting foam was then postcured at 600.degree. F. (315.6.degree. C.) for 15-30 minutes in a circulating air oven.
The third procedure involves use of microwave radiation for converting the polyimide precursor into a cellular structure which normally is then subjected to final curing in a thermal oven. In actual practice the precursor is used in the form of a powder produced by spray drying an alcoholic solution of the tetracarboxylic and diamine components. See, for example, Gagliani et al. U.S. Pat. Nos. 4,296,208; 4,305,796; 4,439,381; and 4,599,365; Final Report NAS 9-15050 (supra); Final Report NAS 9-15484 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Commercial Aircraft and Spacecraft Using a Basic Polyimide Resin" and Final Report NAS 9-16009 entitled "Formulation and Characterization of Polyimide Resilient Foams of Various Densities for Aircraft Seating Applications". In U.S. Pat. Nos. 4,305,796 and 4,439,381 it is indicated that the polyimide precursor may vary from a `liquid resin` to a spreadable, paste-like formulation, depending upon the nature and quantity of any fillers added to the resin.
Precursors which lead to polyimide foam upon heating have been described in various terms. U.S. Pat. No. 3,506,583, which issued in 1970, refers to the precursor as a substantially monomeric "solid solution resinoid." In U.S. Pat. No. 4,900,761 the mixture of substantially equal molar amounts of tetracarboxylic component (generally di-half ester) and diamine component is termed a "paste resin" if the solids content is high and a "liquid resin" if more of the esterifying alcohol is present. U.S. Pat. No. 4,153,783 teaches that free volatiles, e.g., alcohol or water, are detrimental to the foaming process.
The polyimide precursor, regardless of its denomination, has generally been pulverized or ground into a powder. If sufficient alcohol esterifying agent is present or can be added, it has been spray-dried, producing a powder. The powder is then spread as evenly as possible into a mold or onto a substantially flat, horizontal surface which is then heated to foam and finally imidize the powder. However, working with the precursor powder can be hazardous to workmen, because the powder contains amines, many of which are toxic, and the powder fines are easily airborne. In addition, it is difficult to spread the powder uniformly into a mold or onto a surface.
It is known that the foaming of polyimides is related to volatile by-products generated in the amidization and/or imidization reactions. According to U.S. Pat. No. 4,900,761 small changes in the solids content of the polyimide precursor lead to large changes in the density of the resultant foam, but the exemplified changes are random and unpredictable. Application Ser. No. 07/466,122 discloses a process for making polyimide foam whereby a polar protic foam-enhancing material is added to a substantially equimolar mixture of carboxylic and diamine components; the amount added is between about 1/200 and 1/2 parts by weight of the sum of the diamine and carboxylic components. The resultant mixture is termed a slurry. The slurry can be heated, producing a homogeneous "melt," which can then be foamed. Copending application Ser. No. 07/575,982 discloses foams in a range of densities which are produced by controlling the amount of the polar protic foam enhancing-additive which is present in the slurry. The subject matter of the instant application is related to that of the these two earlier applications.